The present invention relates to systems and methods for producing halftone images from digital representations of color images, the image to be recorded as latent or visible images by an output device such as image-setter, plate-setter or a digital printer.
Trappingxe2x80x94creating an overlap (trap/frame) between abutting colors to compensate for imperfections of the printing press.
Anti-aliasingxe2x80x94eliminating visibly jagged steps along angles or object edges, created by sharp tonal contrasts between adjacent color areas.
Screeningxe2x80x94creating a pattern of dots to reproduce color or grayscale continuous-tone images.
Xerographyxe2x80x94An electrostatic non-impact printing process in which heat fuses dry ink toner particles to electrically charged areas of the substrate, forming a permanent image.
In the graphic arts industry, a half tone image, representing the image to be printed as a latent or a visible image, is produced from a digital representation of the image. This digital representation is produced in a workflow that includes two major stages.
In the first stage, performed using an image editing computer, such as a Macintosh, available from Apple Computer Inc. of Cupertino, Calif. and equipped with image-editing software, such as Adobe PhotoShop, available from Adobe Systems Inc. of San Jose, Calif., digital images to be recorded are created and edited by a graphic arts designer. This image editing usually includes one or more page-element specific operations, such as manipulating the colors of an image and preparing a page layout incorporating all the defined page elements in a Page Description Language (PDL).
In the second stage, a series of processes are applied to the input PDL file, resulting in a halftone image to be recorded by a digital front-end (DFE) application. The DFE may be connected to one or more output devices.
FIG. 1 schematically outlines a typical prior-art pre-press to press system, in which an analog picture is scanned (step 10) using a scanner such as the Smart 342, commercially available from Scitex Corporation Ltd. of Herzlia, Israel. This produces a digital representation of the original. The digital file is then edited by the designer in step 12, using software applications such as Adobe PhotoShop, and a page is constructed in step 14, using software applications such as Adobe PageMaker, both available from Adobe Systems Inc. of San Jose, Calif., for color editing and page layout, respectively. The resulting digital representation of a page is converted to a standard file format, such as PostScript, PDF or other page description language.
The standard file is used as an input digital representation to a front-end system 16, connected to an output device 18 that may be an image-setter, a plate-setter, a xerographic digital printer or any other known device for printing. The front-end system such as the Brisque, commercially available from Scitex, produces a half tone image from the input digital representation, after a series of operations, i.e., raster image processing, trapping, anti-aliasing and screening.
FIG. 2 schematically outlines a similar system, with the same basic flow, where the front-end system is connected to several output devices 20 in parallel.
In the prior art configurations of FIGS. 1 and 2, the decision of when and where in the process to perform the second stage operations such as trapping, anti-aliasing and screening is insignificant, Each of the second stage processes requires the page description file as input, as well as precise knowledge of the output device""s characteristics and can be performed anywhere in time, between the page layout stage and the imaging or printing stage.
In one state of the art workflow used by Scitex, the digital page file is rasterized first, to create an intermediate file. The intermediate file goes through an automatic trapping application, such as Full Auto Frame available from Scitex, which automatically analyzes, decides and creates traps where desired, to produce a trapped intermediate file which is then screened. In this workflow, screening is performed in the front-end system, by applications such as Scitex Class Screen or Scitex Turbo Screen, operating on a digital file that has already been trapped.
In another state of the art workflow, trapping is done during rasterization, such as in the In-RIP Trapping application, available from Adobe Systems Inc. of San Jose, Calif. In this workflow, one can define trapping parameters prior to the RIP, to be executed during the RIP process.
In yet another workflow, the trapping functionality is found within the QuarkXpress application, available from Quark Inc. of Denver, Colo. and is performed before conversion of the digital image to a standard page description language file.
There are yet other ways to process the file provided to the front end in a standard file format. Most of them do not take into account the effect that any specific image processing operation might have on the outcome of subsequent image processing operations, in conjunction with a specific output device. For example, the trapping process often results in narrow color areas (trap areas) between specific color combinations of adjacent colors. These tap areas, when imaged by a specific output device, are very sensitive to the imaging capabilities of the output device, in terms of various parameters, including screen resolution, dot size, angle and shape, as well as the actual width of the trap. All of these parameters depend on the type of device that will produce the visible image.
The existing methods for pre-analyzing a file for possible imperfections in printing (resulting from the data of the file and the characteristics of the specific printing device), usually try to prevent those imperfections by modifying the digital data prior to printing, thereby possibly introducing artifacts into the file, enlarging its storage size and creating device-dependency which reduces the flexibility to produce the file on other types of devices, or with different output parameters (i.e. different size, resolution etc.) on the same device.
In general, image processing performed on an image file can produce different results on different output devices, depending on the characteristics of the output device. Thus, in the prior art, the parameters of the specific output device have been used as constraints to the image processing algorithms in order to achieve quality and predictability of output. On the other hand, processing that considers the parameters of a specific output device is less general and requires that the processing be performed separately for each output device.
European Patent Publication EP 0840500 A2 to Adobe Systems deals with the problem of output device dependency by suggesting a method for device independent trapping. According to EP 0840500 A2, the entire trapping algorithm is performed independent of the specific output device, leaving only the determination of the actual trap color to the final printing stage, This solution deals only with the appearance of the trap color and does not address other quality issues as described above.
The present invention provides a workflow for producing improved half tone images to be recorded as latent or visible images by an output device connected to a front-end system.
It is an object of the present invention to provide a workflow divided into two stages; the xe2x80x9cpreparation stagexe2x80x9d, which is output device independent and results in a set of parameters, and the xe2x80x9cproduction stagexe2x80x9d, which involves the actual creation of the visible or latent image on a specific output device, using the parameters created by the preparation stage.
In one aspect, the present invention provides a method for enhancing the quality of a digital halftone image recorded on a substrate:
Initially, at least one color pair comprising two adjacent colors is identified in the digital image data, where there is a difference in color value between said two adjacent color areas in at least one separation of the digital image.
Next, for at least one of the identified pairs, a determination is made whether quality enhancement is desired, based on predetermined criteria which might or might not depend on a specific output device to be used.
Wherever quality enhancement is desired, imaging parameters for modifying the digital image data in the border area between the two adjacent color areas are prepared, based on the previous determination.
During recording, the thus prepared imaging parameters are used to modify the digital image data in the border area between the two adjacent color areas, thereby enhancing the quality of the recorded half tone image.
In another aspect, the present invention provides a method for analyzing a digital image for potential artifacts in the recorded image, by identifying, in the digital image, at least one color pair having two adjacent colors, where there is a difference in color value between the two adjacent color areas in at least one separation of the image. This analysis is performed only once, in a device independent manner.
In yet another aspect, the present invention provides a method for determining, for at least one pair of adjacent colors having a difference in color value in at least one separation, whether quality enhancement is desired, and repeating this determining step once for each potential output device to be used.
In yet another aspect, the present invention provides a method for determining, for at least one pair of adjacent colors having a difference in color value in at least one separation, whether quality enhancement is desired, and specifying the type of quality enhancement for that color pair.
In yet another aspect, the present invention provides a method for modifying imaging parameters of previously indicated adjacent color areas, where the step of modifying may or may not depend on the specific output device to be used.
In yet another aspect, the present invention provides a method for modifying screening parameters, including halftone-dot rotation angle and/or screen resolution, to enhance the quality of an image to be recorded.
In yet another aspect, the present invention provides a method for enhancing the quality of an image to be recorded by a xerographic printer, by decreasing the screen resolution parameter for the border area between two adjacent color areas, to prevent leading edge deletion and/or trailing edge deletion.
In yet another aspect, the present invention provides a method for enhancing the quality of an image to be recorded by an image-setter for subsequent printing on an offset press or by a xerographic printer, by changing the halftone-dot rotation angle parameter for the border area between two adjacent color areas, to prevent jagged edges in the trapping area.
Another aspect of the present invention is to provide a method for preparing information for recording a digital quality enhanced half tone image on a substrate:
Initially, at least one color pair having two adjacent colors, where there is a difference in color value between the two adjacent color areas in at least one separation of the image, is identified in the digital image.
Next, a determination is made, for at least one pair of the at least one color pair, whether quality enhancement is desired, based on pre-determined criteria which may or may not be output device dependent.
Following the determination, imaging parameters are prepared for modifying the image data in the border area between the two adjacent color areas of the at least one color pair, based on the previous determination, and the modified imaging parameters are stored.
It is a further aspect of the present invention to provide a method for enhancing the quality of a digital half tone image recorded on a substrate:
First, an output device is selected from at least one available output devices.
Then, quality enhancement information for the selected output device is read from at least one previously prepared quality enhancement information data.
During recording, the quality enhancement information for the selected output device is used to modify image data, thereby enhancing the quality of the recorded halftone image.
In one aspect of the present invention, the quality enhancement information for the selected output device includes screen parameters, such as halftone-dot rotation angle and/or screen resolution.
In another aspect of the present invention, using the quality enhancement information includes recording a latent or visible image with modified image data on a substrate.